Mobile phone users require quality reception and transmission over a wide area. The quality of the radio frequency (RF) signal depends on the RF filters in the mobile phone. Each RF filter passes desired frequencies and rejects unwanted frequencies enabling band selection and allowing a mobile phone to process only the intended signal.
It has been estimated that by 2020, a shift to Carrier aggregation, 5G and 4×4 MIMO could result in mobile phones requiring upwards of 100 filters and a global market of 200 billion filters a year.
Acoustic resonators are basic building blocks of RF filters and sensors. These typically include a piezoelectric electromechanical transduction layer which converts mechanical energy into electrical energy. These resonators have to be cheap but reliable. The two most common types of acoustic resonators are Surface Acoustic Wave Resonators (SAW) and Bulk Acoustic Wave Resonators (BAW).
In Surface Acoustic Wave resonators the acoustic signal is carried by a surface wave. In Bulk Acoustic Wave Resonators (BAW) the signal is carried through the bulk of the resonator film. The resonant frequency of both types of filter is a characteristic of its dimensions and of the mechanical properties of the materials used in its construction.
The quality of a resonator is given by its Q factor. This is the ratio of the energy stored to the power dissipated. A high Q factor indicates that the filter loses little energy during operation. This translates to a lower insertion loss and a steeper skirt for “sharper” differentiation to nearby bands.
The next generation of mobile phones will be required to operate at higher frequencies to enable transmitting and receiving the ever growing data traffic. Moving to such higher frequencies without enlarging the mobile phone requires small low power resonators that operate at higher frequencies and that can be used in smart phones without rapid depletion of the battery power pack.
The quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, and characterizes a resonator's bandwidth relative to its center frequency. The next generation of mobile phones requires quality resonators having high Q factors.
Bulk-acoustic-wave (BAW) filters provide better performance than surface acoustic wave filters. Whereas the best SAW filters may have Q factors of 1000 to 1500, current state of the art BAW resonators have Q factors of 2500 to 5000.
BAW filters can operate at higher frequencies than SAW filters. They have better power handling, smaller sizes, higher electrostatic discharges (ESD), better bulk radiation and less out of band ripple.
However, SAW filters are simpler and cheaper to manufacture and since the IDT pitch can be varied by the mask layout, resonators having significantly different frequencies can be made on the same die, using the same piezoelectric film thickness.
The electrical impedance of a BAW resonator has two characteristic frequencies: the resonance frequency fR and anti-resonance frequency fA. At fR, the electrical impedance is very small whereas at fA, the electrical impedance is very large. Filters are made by combining several resonators. The shunt resonator is shifted in frequency with respect to the series resonator. When the resonance frequency of the series resonator equals the anti-resonance frequency of the shunt resonator, the maximum signal is transmitted from the input to the output of the device. At the anti-resonance frequency of the series resonator, the impedance between the input and output terminals is high and the filter transmission is blocked. At the resonance frequency of the shunt resonator, any current flowing into the filter section is shorted to ground by the low impedance of the shunt resonator so that the BAW filter also blocks signal transmission at this frequency. The frequency spacing between fR and fA determines the filter bandwidth.
For frequencies other than the resonance and anti-resonance frequencies, the BAW resonator behaves like a Metal-Insulator-Metal (MIM) capacitor. Consequently, far below and far above these resonances, the magnitude of the electrical impedance is proportional to 1/f where f is the frequency. The frequency separation between fR and fA is a measure of the strength of the piezoelectric effect in the resonator that is known as the effective coupling coefficient—represented by K2eff. Another way to describe the effective coupling coefficient is as a measure of the efficiency of the conversion between electrical and mechanical energy by the resonator (or filter). It will be noted that the electromechanical coupling coefficient is a materials related property that defines the K2eff for the piezoelectric film.
The level of performance of a filter is given by its factor of merit (FOM) which is defined as FOM=Q*K2eff.
For practical applications, both a sufficiently high K2eff and high Q factor values are desired. However, there is a trade-off between these parameters. Although K2eff is not a function of frequency, the Q-value is frequency dependent and therefore the FOM (Factor of Merit) is also a function of frequency. Hence the FOM is more commonly used in filter design than in the resonator design.
Depending on the application, often device designers can tolerate a lowering in the K2eff to achieve a high Q factor where a small sacrifice in K2eff gives a large boost in the Q value. However, the opposite approach of sacrificing Q-value to obtain a design having an adequate K2eff is not feasible.
K2eff can be enhanced by choosing a high acoustic impedance electrode, and can also be traded off with other parameters such as electrode thickness and a thicker passivation layer.
There are two main types of BAW resonators (and thus filters): SMR (Solidly Mounted Resonators) and FBAR (Film Bulk Acoustic Resonator resonators.
In the SMR resonator, a Bragg reflector is created under the bottom electrode using a stack of alternating low and high impedance thin film layers, each having a thickness λ/4, where λ is the wavelength of the target frequency. The Bragg reflector stack acts an acoustic mirror to reflect the acoustic wave back into the resonator.
SMR resonators are easier (and thus typically cheaper) to manufacture than FBAR resonators and since the piezoelectric film is attached directly to the substrate, heat is dissipated more effectively. However, in SMR based filters, only the longitudinal acoustic wave is reflected but not the shear waves. Consequently SMR filters have lower Q factors than FBAR based filters.
In the FBAR resonator a free-standing bulk acoustic membrane which is supported only around its edge is used. An air cavity is provided between the bottom electrode and the carrier wafer. The high Q factor of the FBAR is a great advantage over the SMR.
The Commercial FBAR filter market is dominated by Broadcom™ (previously AVAGO™) which uses Aluminum Nitride (AlN) as the piezoelectric thin-film material that best balances performance, manufacturability and Wafer Level Packaging (WLP) processing that employs Si cavity micro-capping over the FBAR device with TSV (through silicon via) for flip chip electrical contacts. AlN has the highest acoustic velocity for a piezoelectric film (11,300 m/s) and hence requires a thicker film for a given resonance frequency which eases process tolerances. Furthermore, high quality sputtered AlN films with FWHM (Full width at half maximum XRD peak) of less than 1.8 degrees allow K2eff values that are above 6.4% which is conveniently about twice the transmit band for FCC mandated PCS. With Q values reaching 5000, FOM values of 250 to 300 are achievable, representing best in class filter devices. K2eff must be kept constant to meet the band requirement. Consequently, to improve the FOM of a filter generally requires increasing the Q value.
Despite the high performance of the above mentioned FBAR filters, issues still remain that prevent moving forward to the next generation of wireless communication. The greater number of users sending and receiving more data results in increasingly jammed bands. To overcome this, future bandwidths should be more flexible to adapt to agile arrangements of different bands. For example, The 5 GHz WiFi band has 3 sub-bands located at 5.150-5.350 GHz, 5.475-5.725 GHz, 5.725-5.825 GHz, respectively, corresponding to required K2eff of around 7.6%, 8.8% and 3.4%. The coupling coefficient K2eff is mainly decided by the intrinsic nature of the piezoelectric material, but is affected by the crystalline quality and orientation of the piezo film, by exterior capacitors and inductors and by the thickness of the electrodes and other stacked materials. The bandwidth of AlN FBARs is mainly modulated by inductors and capacitors that are pre-integrated into the IC substrate carriers. However, these elements degrade the Q factor and also increase the substrate layer count and thus increase the size of the final product. Another approach for K2eff modulation is to use an electrostrictive material to realize tunable band FBAR filters. One candidate material is BaxSr1-xTiO3 (BST) that may be tuned once the DC electrical field is applied.
Tunability with BST can also be achieved by using the material as a variable capacitor build in part with the FBAR resonators circuitry thereby assisting in matching filters and in adjusting their rejection. Furthermore, since a BST FBAR resonates only with a certain applied DC bias voltage, it may represent low leakage switching properties, potentially eliminating switches from the Front End Module (FEM) of the mobile device and thereby simplifying module architecture and reducing both size and cost. BST FBARs also possess other favorable properties for RF applications. The high permittivity of ferroelectric materials (εr>100) allows for reduction in the size of devices; for example, a typical BST resonator area and BST filter area is in the order of 0.001 mm2 and 0.01 mm2, respectively, at low GHz frequencies in standard 50-Ω RF systems. In fact, using BST the resonator size may be an order of magnitude smaller than that of conventional AlN resonators. Moreover, the power consumption in the BST FBAR itself is negligible—even with the usage of the above-mentioned DC bias voltage across the device due to a very small leakage current in the BST thin-film.
Strong c-axis texture is the most important prerequisite for AlN or BST based FBARs because the acoustic mode for such FBARs needs to be longitudinally activated, and the piezoelectric axis of both AlN and BST is along the c-axis. Hence high quality single crystal piezo film, as represented by FWHM of less than 1°, have great impact on the FBAR filter properties and can reduce the RF power that is otherwise wasted as heat by as much as 50%. This power saving can significantly reduce the rate of drop calls and increase the battery life of mobile phones.
Epitaxial piezoelectric films with single orientation may have other merits. For example, strongly textured epitaxially grown single crystal piezo films are expected to have smoother surfaces than those of randomly oriented films. This in turn, results in reduced scattering loss and a smoother interface between the metal electrodes to the piezo films which both contribute to a higher Q-factor.
Furthermore, there is an inverse thickness to operating frequency relationship for AlN and BST filter films. Ultra thin-films are needed for extremely high frequency filters such as 5 GHz WiFi, Ku and K band filters. For filter operating at 6.5 GHz the thickness of BST film should be around 270 nm and for 10 GHz the thickness of an AlN film should be around 200 nm. These dimensions invokes serious challenges for film growth because it is hard to attain the necessary stiffness for an extremely thin anchored membrane and the crystalline defects and strains are more likely to cause cracks and mechanical failures as the membrane film becomes thinner. As such, more innovative membrane supporting structures with defect-free single crystal films are needed for the next generation of high frequency FBARs.
Unfortunately, AlN, BST and other piezoelectric materials have vast lattice spacing and orientation differences to those of currently used bottom electrode metals. Furthermore, the range of bottom electrode materials available, especially in the case of BST, is very limited since they have to withstand relatively high temperatures during the subsequent deposition of the piezo film thereupon. As a result, to date, no true high quality single crystal piezo FBAR films have been successfully demonstrated.